LANTR, or how to fix NTRs without LF-only tanks.

[SargeRho]

So, the idea of NTRs burning both fuel and oxidiser is not as unrealistic as one may think. I've read about the idea of pumping oxidiser into the exhaust flow of an NTR as a kind of afterburner, called LANTR - LOX-Augmented Nuclear Thermal Rocket. The mechanics behind the RAPIER engines would be applicable here. In pure nuclear mode, the NTR only burns liquid fuel, in Oxidiser-augmented mode, its Isp is severely reduced, but it gets a significant boost in the thrust department (from 60 to 120 perhaps?).

This would remove the need to have more liquid fuel-only tanks to make NTRs work as they should, and makes them more Kerbal at the same time. I mean, what is more Kerbal than a nuclear rocket engine with after burner? Other than Orion of course.

[Starwaster]

So, the idea of NTRs burning both fuel and oxidiser is not as unrealistic as one may think. I've read about the idea of pumping oxidiser into the exhaust flow of an NTR as a kind of afterburner, called LANTR - LOX-Augmented Nuclear Thermal Rocket. The mechanics behind the RAPIER engines would be applicable here. In pure nuclear mode, the NTR only burns liquid fuel, in Oxidiser-augmented mode, its Isp is severely reduced, but it gets a significant boost in the thrust department (from 60 to 120 perhaps?).

This would remove the need to have more liquid fuel-only tanks to make NTRs work as they should, and makes them more Kerbal at the same time. I mean, what is more Kerbal than a nuclear rocket engine with after burner? Other than Orion of course.

There are three problems with that assumption. Pick two.

• Propellant density.
  
• Specific impulse rating
  
• Thrust level.
  

The stock LV-N has 60 kN of thrust, 800 Isp and 5kg per volume unit for both fuel and oxidizer (0.005 tons per volume unit). I'll leave aside the argument as to what the volume unit is but simply put that cannot possibly match the stats of a LANTR which uses liquid hydrogen + liquid oxygen as its propellants. The problem here is that liquid hydrogen in a typical NTR has an Isp of 850-910. Somewhat higher Isp is possible. Thrust of 60 kN is on the low end for a smaller NTR but within range if we're only talking about hydrogen as the propellant

In a LANTR, liquid oxygen is injected into the nozzle into the supersonic exhaust stream right after it exits the throat. This has the effect of lowering Isp and raising thrust. The exact amounts depends on ratio of LOX/H2. Ratios of 1.0 to 7.0 have been calculated which drops Isp to the low 700s for a ratio of 1.0 (LOX to H2 by mass, not by volume) and into the low 500s up to a ratio of 7.0. Thrust would be increased into the mid to high 500s.

The numbers just aren't even really close to convince me that the LVN is actually operating in LANTR mode. Isp is too high and thrust is too low. (Edit: and propellant is too dense. Low density exhaust = higher Isp. Higher density exhaust = lower Isp. Thrust tends to be higher for high density propellant because it's based on exhaust velocity x mass flow)

This page here has some decent information the data table has been floating around for years in various proposals for NTR usage.

http://www.nss.org/settlement/moon/LANTR.html

Edited January 22, 2014 by Starwaster

[SargeRho]

You missed my point. By default it'd only chugg liquid fuel, and all other stats would be identical to how it is now, but you can switch to LANTR mode which has a higher thrust, lower ISP and consumes oxidiser.

[GluttonyReaper]

You missed my point. By default it'd only chugg liquid fuel, and all other stats would be identical to how it is now, but you can switch to LANTR mode which has a higher thrust, lower ISP and consumes oxidiser.

Interesting idea, but how would this actually be useful, gameplay-wise? Isn't the whole point to have a really efficient engine?

[Wahgineer]

Interesting idea, but how would this actually be useful, gameplay-wise? Isn't the whole point to have a really efficient engine?

Maybe when money is introduced, it would allow you to get the same power with less engines? That would be nice, especially if you could adjust how much oxidizer it uses and it doesn't reduce the ISP too much.

[Anton P. Nym]

Interesting idea, but how would this actually be useful, gameplay-wise? Isn't the whole point to have a really efficient engine?

If you suppose a LANTR using RP-1 instead of LH, then in-game you could save on mass by using the existing Jet Fuel tanks (that don't also carry oxidiser) for your big burns. Switching to the higher-thrust KerLOX mode would require current rocket tanks.

I think the suggestion is mainly workable; the only sticking point I see is the switch-over from high-thrust to high-efficiency mode, but making it like the RAPIER would help. (Come to think of it, something similar would also make VASIMIR engines workable.)

-- Steve

[SargeRho]

It could serve as a heavy multi-role engine. Sort of like the RAPIER, but for space. When you need a high efficiency rocket engine, you use it in its regular mode. If you need a bit more thrust for whatever reason, for example for a nuclear upper stage, use the LOX Augmented mode.

[coolitic]

I would use this to make missiles go faster when they are about to hit the target.

In realistic use, maybe use oxidizer when near surface and efficient mode when in space?

[Starwaster]

You missed my point. By default it'd only chugg liquid fuel, and all other stats would be identical to how it is now, but you can switch to LANTR mode which has a higher thrust, lower ISP and consumes oxidiser.

It's not that I missed the point. There is no way possible that the resource LiquidFuel would give a high Isp. To be at all realistic LiquidFuel needs replacing with a propellant that is much less dense and requires a higher volume tank to contain the equivalent mass. Isp is derived from exhaust velocity which in turn is derived from exhaust molecular mass. That's why H2 gives a high Isp by itself. There's nothing realistic about the situation and it can't be made realistic just by slapping a RAPIER mode in there and swapping out thrust / Isp. Assuming that the relevant module can provide that functionality. I haven't looked at it but I'll assume since you brought it up that it is possible. It changes nothing

It's just swapping one magic engine for another.

For those who use Real Fuels btw realistic NTR engines are available. Not everyone likes the realism involved but that's another can of worms entirely.

[Anton P. Nym]

It's not that I missed the point. There is no way possible that the resource LiquidFuel would give a high Isp.

Actually you can... ISP scales by mass and the *square* of the velocity (e=mv^2/2); by using a nuclear energy source you can get a fairly high ISP using RP-1 if you needed to. It's just that LH will get a higher ISP for the same energy output because of its lower mass.

-- Steve

[Starwaster]

Actually you can... ISP scales by mass and the *square* of the velocity (e=mv^2/2); by using a nuclear energy source you can get a fairly high ISP using RP-1 if you needed to. It's just that LH will get a higher ISP for the same energy output because of its lower mass.

-- Steve

If I had time I'd go into the actual formula, which involves square roots and not squares.... to determine exhaust velocity which is where you get Isp from.

The bottom line is that all other factors being equal, your Isp will always be lower for an exhaust with high molecular weight compared to one with low molecular weight. Always. Increasing mass could increase thrust if you're talking about mass flow, but just increasing the density of the propellant will lower Isp.

If you want you can increase chamber temperature until your propellant completely disassociates into its atomic components which would help but none of that applies to the rocket motor being discussed which is derived from researched technology whose abilities and limitations is very well known.

[Aghanim]

It's not that I missed the point. There is no way possible that the resource LiquidFuel would give a high Isp. To be at all realistic LiquidFuel needs replacing with a propellant that is much less dense and requires a higher volume tank to contain the equivalent mass. Isp is derived from exhaust velocity which in turn is derived from exhaust molecular mass. That's why H2 gives a high Isp by itself. There's nothing realistic about the situation and it can't be made realistic just by slapping a RAPIER mode in there and swapping out thrust / Isp. Assuming that the relevant module can provide that functionality. I haven't looked at it but I'll assume since you brought it up that it is possible. It changes nothing

It's just swapping one magic engine for another.

For those who use Real Fuels btw realistic NTR engines are available. Not everyone likes the realism involved but that's another can of worms entirely.

Actually I likes RealFuels realistic NTR, because a tank full of H2 is much lighter than tank full of kerolox